Activation of the fibrinolytic system is required to restore blood vessel patency after coagulation and occurs in processes such as wound healing and inflammation in which proteolysis may regulate the migration of participating cells through tissues. Activation of this system results in conversion of plasminogen to plasmin, a broad spectrum serine protease. Perturbation of this system can have pathological results ranging from thrombosis to hemorrhagic diatheses. Known mechanisms for regulating the fibrinolytic system include the availability of plasminogen activators, protease inhibitors and fibrin. The overall objective of this proposal is to test the hypothesis that cell surface receptors for plasminogen constitute a unique and previously unrecognized mechanism for regulating the fibrinolytic system. Specifically, the functions of plasminogen receptors on a cell found predominantly in the circulation, the platelet, and a cell which migrates into the extravascular space, the monocyte, will be evaluated. Plasminogen binds specifically and saturably to platelets and is preferentially activated on the cell surface. The ligand also binds to monocytes and the monocyte-like cell line, U937 and its binding is enhanced by the plasminogen activator, urokinase. The first aim of the proposal is to test the hypothesis that plasminogen binding to platelets enhances the catalytic efficiency of plasminogen activation. The second aim is to test the hypothesis that the plasminogen binding site on the monocyte is functionally similar to that of unstimulated platelets and that this site can be regulated in concert with the expression of monocyte plasminogen activator activity. For both cell types, the hypothesis that cell-bound plasmin is protected from inhibition and that plasmin affects cellular functions will be tested. For both cell types the experimental objectives include: (1) comparison of kinetic parameters for activation of cell-bound and soluble plasminogen; (2) comparison of rates of inactivation by protease inhibitors of cell-bound versus soluble plasmin: (3) identification of the region(s) of plasminogen involved in the interaction; and (4) assessment of functional consequences of plasmin formation on the cells. Additional experimental objectives for the monocytes are: (1) characterization of the interaction of plasminogen with these cells; (2) identification of the plasminogen binding site; (3) assessing regulation of plasminogen receptors by monocyte agonists, urokinase and the urokinase receptor. The results obtained will provide basic understanding of the regulation of fibrinolysis by and on cell surfaces and may also suggest mechanisms by which the fibrinolytic system may be manipulated for therapeutic purposes.